Aluminum is produced conventionally by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte bath at temperatures between about 900° C. and 1000° C.; the process is known as the Hall-Heroult process. A Hall-Heroult reduction cell typically comprises a steel shell having an insulating lining of refractory material, which in turn has a lining of carbon that contact the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate that forms the cell bottom floor. The anodes are at least partially submerged in the cryolite bath.
Electrolytic reduction cells must be heated from room temperature to approximately the desired operating temperature before the production of metal can be initiated. Heating is done gradually and evenly to avoid thermal shock, which can in turn cause breakage or spalling. The heating operation minimizes thermal shock to the lining, the electrodes and the support structure assemblies upon introduction of the electrolyte and molten metal to the cell. Once at operating temperatures carbon anodes erode and have to be replaced usually one at a time, in what is called a “change out” operation. D'Astolfo Jr. et al. in U.S. Pat. No. 6,551,489B2 addressed change out operations where an inert anode assembly containing from about four to eleven inert anodes on a common conducting support was used to replace standard single, large carbon anodes. The inert anodes were about from 12 cm to 76 cm. in diameter and from about 12 cm. to 38 cm. high.
Carbon anodes can be placed in to the electrolyte cold and heated by the energy of the cell to operating temperatures, at which time the nominal current of the anode will be attained. Ceramic anodes have much longer lives but are more prone to thermal shock and therefore need to be preheated in a furnace outside of the electrolytic cell prior to insertion into the hot electrolyte. During transfer, the cooling or heating of the anodes must be also minimized to avoid thermal shock. The thermal shock/cracking was thought to only occur both during movement of the anodes into position and during their placement into the molten salt. Thermal shock relates to the thermal gradient (positive or negative) through the anode that occurs, usually during the movement from the preheat furnace to the cell, and also upon insertion of the anodes into the molten salt. Depending upon the time frame, a thermal gradient as low as between about 20° C. to 50° C. can cause cracking.
In an attempt to protect electrodes in an electrolysis cell from thermal shock during start-up, U.S. Pat. No. 4,265,717 (Wiltzius), taught protection of hollow cylindrical TiB2 cathodes by inserting aluminum alloy plugs into the cathode cavity and further protecting the cathode with a heat dispersing metal jacket having an inside heat insulating layer contacting the TiB2, made of expanded, fibrous kaolin-china clay (Al2O3.2SiO2.2H2O), which would subsequently dissolve in the molten electrolyte. In U.S. Pat. No. 6,447,667 B1 (Bates et al.) the inert anode was coated with carbon and/or aluminum as protection against the cryolite bath. Also, in U.S. Patent Application Publication No. 2003/0127339A1 (LaCamera et al.) anodes were first heated and had an insulating boot attached during submersion into the molten bath. A silica or alumina insulating material was found to be effective. However, such silica or alumina boots were made to dissolve in the bath over time, so that at change out they would usually be non-existent.
Aluminum electrolysis cells have historically employed carbon anodes on a commercial scale. The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable, and dimensionally stable anodes. Use of inert anodes rather than traditional carbon anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also realized because inert anodes produce essentially no CO2 or CF4 emissions. Some examples of inert anode compositions are provided, for example, in U.S. Pat. Nos. 4,374,761; 5,279,715; and 6,126,799 assigned to Alcoa Inc.
It has recently been found, that, in inert anodes cells, when an anode is replaced by taking it out of an operating bath, at about 960° C., its function as a “heat sink” and radiation shield is lost and the surface temperature of exposed adjacent inert anodes still operating in the molten bath can drop more than 25° C. during the first minute. This could cause adjacent inert anodes to crack and fail in the first 20 seconds. This problem has created a critical need to protect the anodes remaining in the molten bath from temperature drops during change out. It is therefore a main object of this invention to provide some means to protect inert anodes from such temperature drops.